Conventionally, a magnetic disk unit has been widely used as a memory of an information processing apparatus. However, a magnetic disk unit is weak against shocks because a highly precise driving mechanism is contained. Also, since a recording medium is mechanically accessed, no high-speed access is possible.
Recently, therefore, the development of a semiconductor memory is being sought as a memory of an information processing apparatus. A semiconductor memory is strong against shocks because no driving mechanism is contained, and high-speed access is possible because data is electrically read out.
With the recent progress of semiconductor technologies, particularly the progress of micro-patterning technologies, micropatterning of memory cells, i.e., high integration of semiconductor memories has rapidly advanced. This poses a problem concerning the storage characteristics of a memory cell.
For example, in a DRAM in which a memory cell is constructed by connecting a MOS transistor and a capacitor in series, the capacitance tends to decrease with a reduction in the capacitor area caused by high integration. This results in a soft error, i.e., false memory contents are read out, or stored contents are destroyed by .alpha.-rays.
To solve this problem, it is important not to reduce the capacitance although memory cell geometries shrink. To this end, it is essential to decrease the thickness of a capacitor insulating film as well as to increase the capacitor area. An example of a method of forming a thin insulating film is to form a silicon nitride film by deposited a silicon film using LPCVD method after thermally nitriding a native oxide film on the surface of a silicon substrate.
Unfortunately, a silicon nitride film formed by this method has a large leakage current and hence is unsuitable for a capacitor insulating film. From this viewpoint, it is presumably effective to form a capacitor insulating film devoid of a native film in order to further decrease the thickness of a capacitor insulating film.
One capacitor insulating film formation method taking account of this problem removes a native film by high-temperature annealing in an ambient containing hydrogen or silane and forms a silicon nitride film by thermal nitridation or chemical vapor deposition (CVD).
If a native film is removed by high-temperature annealing, however, an impurity in an impurity diffusion layer (one capacitor electrode) formed on the surface of a silicon substrate outdiffuses and lowers the impurity concentration in the impurity diffusion layer. This lowers the capacitance of a depletion layer formed in the silicon substrate. As a consequence, the electrical capacitance lowers, and the apparent leakage current increases.
Another native film removing method uses hydrofluoric acid vapor. This method has the advantage that a native film can be removed at a low temperature. However, hydrofluoric acid remains on a silicon substrate. Therefore, when a silicon nitride film is formed by CVD, the thickness of this silicon nitride film varies microscopically because of the low nucleus density along the surface of the sillicon substrate, and surface roughness increases. Consequently, a thin film portion is formed, and the leakage current increases in this portion.